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Homework Help: How I can find time by this formula S=ut+1/2at^2

  1. May 17, 2012 #1
    Hi all

    I have

    s = 48 m
    u = 30 m/s
    a = -9.37


    S = ut +1/2at^2

    48 = 30t + 1/2( -9.37 ) t^2

    Now after that , what should I do ?
  2. jcsd
  3. May 17, 2012 #2


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    Homework Helper

    Algebra. This is a quadratic equation. Put it into standard form and then find the roots for t (i.e. used the quadratic formula, but with t instead the of the traditional x).
  4. May 17, 2012 #3
    we havew done this sort of calc. Do you know the x =[-b +/- sqrt(b^2-4ac)]/2a equation ?
    before you use this always check the b^2-4ac value. It tells you what sort of answer youwill get.
    In this question I got b^2 - 4ac to be =1 which means there is only one answer and quite easy to work out (b/2a)
  5. May 17, 2012 #4
    turn 48 = 30t + 1/2( -9.37 ) t^2 into 4.685t^2+30t-48=0 (turn a in positive) wich is analog to ax^2+bx+c=0
    using x =[-b +/- sqrt(b^2-4ac)]/2a you'll find: t1=1.33 and t2=-7.73.
    the right answer is t1 bacause time can't be negative
  6. May 17, 2012 #5
    thats wrong. If you use 1.33 and notice the - sign for acceleration....it does not give an equal answer.
    30/9.37 = 3.2 is correct = (b/2a)
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